The separation of fats and oils from oil-containing natural edible raw materials of various types and the use of the extracted fat and defatted residue as food ingredients is a time-honored practice in food preparation. Olive oil, produced by pressing olives at certain ripeness, dates back at least as far as written history, and in several parts of the world continues to this day to be produced in the time-honored manner. Over the course of human history, many other sources of oil were discovered and developed, including sunflower, safflower, sesame, nutmeat and grape seeds.
Throughout history, oil has been a valued adjunct in cooking by virtue of its ability to impart flavor and mouth-feel to food, promote evenness of cooking, and retard food degradation over time. Fats and oils have also been prized for nutritional reasons because they contain more than double the energy content, i. e. about 9 Kcal/gm, of carbohydrates and proteins. Also, many fats and oils contain essential fatty acids which the human body cannot endogenously produce and yet needs for proper function.
Nutritional trends over the past several decades in the United States other industrially developed countries indicate that for the most part, inadequate caloric intake is no longer a nutritional problem. Indeed, the very reverse appears to be true in that for the United States in particular, a startling increase in obesity has been observed over the last two decades. This trend has aroused concern among public health officials and has led to the creation of a large and rapidly growing diet food industry, increasingly restrictive changes in food labeling regulations, and major changes in dietary recommendations by food authorities.
Past innovators in the area of extracting fats and oil from seeds (including nut kernels or meats) have concentrated on maximizing the amount of high-quality, fat recovered without regard to damage done to the structure of the seed matrix, which was often deliberately sacrificed in order to increase efficiency and because maintaining structural integrity of the residual matrix has relatively little importance, such residual matrix being used principally as cattle feed. Thus, the seeds or meats were comminuted or even finely ground to facilitate a rapid and high yield extraction so that the sub-divided residue completely lost its natural coherent structure. Consequently, the only nutmeats available in reduced fat form were comminuted or ground.
Due at least to an important extent to the convergence of the latter practice with the above trends, seeds in nut or shelled form, e.g. peanuts, tree-nuts and the like, have suffered a definite decrease in consumption in the United States. It is a reasonable conclusion that more peanuts, cashews, and other nut products would be consumed if high quality, reduced-fat versions of the same were available. Obviously, the ideal nut snack or candy additive for many people would have the same texture, flavor, and appearance of a normal full-fat nut, but with appreciably less fat.
This invention relates to a process for extracting by means of propane, butane, and/or isobutane as solvent large amounts of fat from edible products which either retain their natural structure in entirety or have been relatively lightly sub-divided into marketable granules of fairly large physical dimension which are characterized by a substantially intact cellular structure. The present process has produced, among other products, partially defatted whole peanuts, partially defatted whole cashews (and other nuts), and partially or wholly defatted nutmeat granules and pieces. By leaving the cellular structure essentially intact after extraction, i.e. by neither finely (comminuting nor reconstructing a finely divided form into a granular product, the texture and appearance of the starting raw material can be preserved but with a reduced fat content.
The primary concern of the present invention is to produce from oil- or fat-containing edible raw material, without the use of any toxic substances, partially and/or fully defatted nut products which are substantially similar in shape and structure to the starting raw material, and have acceptable texture and flavor as compared with the raw material, yet with significantly less fat or oil content.
Historically, methods for recovering vegetable fat and oil can be separated into three general time periods:
(a) Pressing with a fixed mechanical or hydraulic press was the predominant approach from the earliest days to the latter part of the 19th century, and is still in limited use today for the production of some very high value oils. This technique was restricted to batch operation and was never suitable for large-scale production. Moreover, oil yields are limited by the achievable pressure, and residual oil levels in press-cake are often in the neighborhood of 20% or even more, though lower values are sometimes possible. The raw material is usually, but not always, broken up into fine meal under the pressures generated in the press. When not broken up into fine meal, the pressing operation results in a grossly distorted flattened product which, even after puffing or re-expansion, still shows evidence of folds or creases on the surfaces resulting from the pressing operation.
(b) Expelling, e.g. with a continuously operating screw or the like, was initially developed late in the last century and grew into a major form of processing in the first half of the current one. It continues to be used extensively around the world today and improvements to the technology continue to be made. It lends itself very well to large economies of scale and continuous production, resulting in low unit costs. Often the raw material is toasted prior to or during the expelling process in order to coagulate proteins and better release the oil therefrom. The raw material is invariably ground into meal by the process. Residual oil contents are lower than for pressing, often reaching as little as about 10%.
(c) Solvent extraction of oil was originally developed in Germany between the two world wars in order to maximize the yield of oil removed from an oleaginous material. In this process a product which may have been already partially de-oiled by expelling is often used as raw material. In that case, the product is substantially comminuted. If not comminuted as a consequence of a prior partially defatting treatment, the raw material is comminuted before direct extraction is carried out. Usually, the raw material is flaked or ground and pelletized (to minimize fines) and is then contacted with a solvent which is most frequently hexane. The solvent dissolves more than 90% of the fat present in a matter of a few minutes. The solvent is then separated from extracted oil by desolventization (usually by means of heat), while the totally defatted meal is delivered into a desolventizer/toaster where it is subjected to a heat treatment to boil away remaining solvent. Processes of this type are used extensively throughout the world. They yield oil of edible quality and meal useful as animal feed, both products playing critical roles in the world food supply.
Some fatty commodities are processed in very unique ways, as, for example, cocoa, which is often converted first into chocolate liquor, part of which is subsequently pressed for separation into cocoa butter and cocoa powder.
Historically, virtually all processes for recovering vegetable fat and oil have begun with raw material which is either fully comminuted in advance of processing or undergoes comminution during the extraction process itself. The historical reason for this is twofold. First, research experience has confirmed ordinary common sense that penetration of a normally liquid solvent into a non-comminuted raw material is at best difficult to achieve. One report of such experience may be found in Othmer et al. Chem. Eng. Prog. 51:372 (1955), but many other like articles are in the technical literature.
One problem is that nutmeats like all living matter have a cellular structure where cells are separated by walls or membranes which resist solvent action. In the first place, the integrity of the wall itself resists solvent penetration. Secondly, even if a wall is ruptured, migration of solvent into, and of the oil-loaded solvent out of, a consecutive series of cells (which may run into the thousands) necessarily depends upon a slow diffusional process. When an oil-containing body such as a whole peanut is contacted with a solvent such as hexane, the first oil, present in the outer layers of cells, is reached relatively quickly by the solvent but there is nothing to force the solvent through intact cell walls into the interior to extract the oil from interior cells. The historical experience has therefore green that solvent extraction on non-comminuted large, and particularly whole, pieces of nut material has resulted in abysmally low oil yields, and those trained in the art have concluded that only surface oil can be extracted therefrom. A second problem encountered early on in the extraction of oil from natural materials is the extreme difficulty in removing residual normally liquid solvent from the processed material. Hexane, which is almost universally used as a solvent for oil, is a known mutagen and causative agent for kidney problems and central nervous system disorders. It is also a suspected carcinogen. Its use is subject to severe legal restrictions which require that the desolventized edible products be completely free of solvent residue. While this is fairly straightforward to do from the oil and from completely comminuted fine meal, it is virtually impossible to achieve from defatted nut meats of intact cell structure. The use of hexane, and most other conventional solvents, is therefore rendered impractical for products of defined natural shape and cell structure.
Historically, the solution to these problems has been fairly simple and focused on raw material preparation. Countless proposals have been made over the past five decades for comminuting, toasting, drying, flaking, milling and pelletizing and so on to prepare the raw material for extraction and desolventization. Soybeans and peanuts, for example, are most commonly flaked or sometimes ground and pelletized, and modern mills use mixtures of flakes (which are the easiest to desolventize) with pellets (which keep the extraction beds fluffy and allow for easy solvent percolation and drainage). The purpose for preparation of the raw material for extraction by comminution by grinding, shearing and the like is clearly to reduce the effective particle size (i.e. minimum dimension) to dimensions allowing easy penetration of solvent. The minimum dimension required for this has almost invariably been on the order of thousandths of an inch or less. Even when the preparation process resulted in pellets as large as xc2xc or xe2x85x9c inch diameter and an inch in length, these pellets were made up of very fine particles whose minimum dimension is on the order of microns. In all cases, from flaking to pre-pressing to grinding, the minimum particle dimension described in the prior art is on the order of a few cell diameters or less.
One important aspect of this invention differentiating it from the prior art is its applicability to particles which remain in their natural basic structural state, so that even if chopped to a desirable market granulation, the minimum particle dimension in terms of intact cell diameters is orders of magnitude higher than is the case in the prior art. With a typical cell diameter of around 10 microns, and arbitrarily taking as a minimum particles of, say, 50 cell diameters, then 500 microns particle minimum dimension is the lower limit of the application of the present invention could conceivably begin, broadly speaking, at about 0.5 mm or {fraction (1/50)} inch minimum particle dimension. But in practical terms and for sake of simplification, the invention is deemed applicable to granular material of the type under discussion having a minimum dimension of {fraction (1/32)} inch, and more typically a minimum dimension of about {fraction (1/16)} inch, within which the location of one cell relative to other cells in the matrix does not vary from their natural state.
The use of propane, butane, and isobutane (for convenience xe2x80x9cPBIxe2x80x9d), as solvents for oil in extraction processes where the raw material is converted to finely divided form can be found in many disclosures, starting with Rosenthal U.S. Pat. No. 2,254,245 and Reid U.S. Pat. No. 1,802,533 followed by Leaders U.S. Pat. No. 2,548,434, and continuing up to the present as evidenced by Heidias U.S. Pat. No. 5,405,633, Franke U.S. Pat. No. 5,281,732 and Zosel U.S. Pat. No. 4,331,695. The inventions in the prior art have all focused on the use of propane, butane, and isobutane for extracting oils from seeds such as soybeans, cottonseed, sunflower seed, and the like in comminuted systems, referred to as either xe2x80x9crolledxe2x80x9d, xe2x80x9ccakexe2x80x9d, or xe2x80x9cflakedxe2x80x9d. Indeed, Miller U.S. Pat. No. 2,687,551 reveals a method of gaining access to oil by using the force of the PBI solvent itself in a gun to destroy the cell structure, in addition to its well-known solvent action for the oil to be extracted. In cases where comminution was not carried out as a previous step in the process, the product undergoing extraction was naturally involved particulate matter of very small dimension. For example, rice bran is a by-product of rice milling and naturally comes off the milling machine with average particle size on the order of 80-100 mesh.
Following the logic outlined above regarding nutritional trends, considerable effort has been expended on methods for preparing low-fat nut derivatives. Vix et al U.S. Pat. No. 3,294,549 described a method for reducing the fat of nutmeats by a process of hydraulic pressing. Various improvements to the process have since been made, for example by Holloway U.S. Pat. No. 4,329,375 and subsequently by Wong et al. U.S. Pat. No. 5,164,217, principally for the purpose of improving the visual quality of the nut granules by reducing the amount of cracked material as well as the proportion of severely distorted nut granules. Subjecting a nut to pressures high as 7,000 psi in a hydraulic cage causes it to become substantially flat, and the latter patents cited above offer various creative ways for inducing the flattened nut to re-expand, preferably to a more or less oval shape which bears some resemblance to the starting nut. However, in industrial (and even laboratory) practice, it is virtually impossible to cause a mass of nuts to re-expand to their original shape. In particular, I have taken magnified photographs of the surface of commercially available nuts; partially defatted by means of mechanical pressure and then subjected to some expansion treatment reveals unevenness at the surface and often wrinkles as well. This distortion in appearance undoubtedly explains why nuts marketed after defatting in this manner have been xe2x80x9choney-roastedxe2x80x9d which leaves on the nuts a coating that hide the deformation and distortion in their appearance.
Other improvements made in the pressing-type operations (for example, Holloway et al. U.S. Pat. No. 4,329,375, Wilkins et al. U.S. Pat. No. 4,466,987, Gannis et al. U.S. Pat. Nos. 4,938,987 and 5,002,802, and Zook et al, U.S. Pat. Nos. 5,094,874 and 5,240,726) have mainly centered on methods of infusing flavors into and improving texture of the partially defatted products produced by pressing.
In addition to their unnatural appearance, the products resulting from a pressing process are chemically dissimilar from the corresponding components of the original nuts as well. Pressing operations on peanuts, for example, remove unsaturated fat preferentially from the nut-meat. Thus, analysis of the oil remaining within the nutmeat after defatting by means of pressure gives a fatty acid profile higher in saturated fatty acids than the original oil present therein, while the oil extracted during the pressing operation contains relatively slightly less saturated fat. The amount by which saturated fat remains preferentially in the partially defatted nutmeats is significant enough to deter manufacturers from making certain saturated fat-related health claims, e.g. xe2x80x9clow in saturated fatxe2x80x9d, for the processed nutmeats under current food labeling regulations.
Recently, Passey et al. U.S. Pat. No. 5,290,578 described a process for making reduced fat peanuts using carbon dioxide under supercritical conditions. In their invention, there was apparently some solvent penetration through cell walls into the interior of the peanuts. The pressures specified here were between 27 Mpa and 41 Mpa, or approximately 4,000 psi to 6,000 psi. Analysis of the data presented shows that even under such tremendous pressures (disregarding the practical difficulty in achieving such pressures on an industrial scale which would require an extraction vessel with steel walls more than 12 inches thick), and after 12 hours of extraction, and at solvent-to-feed ratios of at least 15 pounds of carbon dioxide per lb peanuts, in the best experiment reported, about 32% of oil present was recovered from the extractor (i.e., on a volumetric basis, e.g. lbs/cu. ft.). Assuming a 50% fat content in the incoming peanuts, this means that the finished peanuts, in the best of experiments reported, contained about 42% fat, giving less than a 20% by weight reduction. Nor is there any disclosure or evidence to suggest that a peanut with a lower fat content than that could be achieved.
Moreover, a fairly complicated multi-step pre-treatment involving humidification and microwaving was used for the best extraction results and to avoid breakage. In fact, for best results it was necessary for the nuts to be first humidified, then microwaved, then re-humidified, then re-microwaved, and finally re-humidified for a third time. One trained in the art of desiccation would conclude that the intent of this sequence was to essentially destroy the cell structure of the nuts prior to the CO2 extraction. Such combinations of humidification and desiccation are well known to cause an opening of cell structure by denaturation of proteins and retrogradation of starch, particularly when carried out at the reported high temperatures (158 F.) and long times (7 hours) for each cycle.
The reduction in the amount of broken peanuts during extraction reported in the disclosure is therefore understandable on the basis of my experimental evidence which shows that when non-comminuted nutmeats are subjected to normally gaseous fluid under high pressure and then subjected to rapid de-pressurization to atmospheric, the nutmeats may explode. During depressurization, there remains within the structure of the meat pockets of high-pressure fluid which creates a stress on the meat as the fluid attempts to escape to the surrounding low pressure atmosphere. Carbon dioxide has an extremely high vapor pressure at room temperature and has the capacity to cause extreme stresses on the nutmeat which cannot release the pressure contained in these pockets of gas quickly enough, thereby causing the nutmeat to fissure and/or explode. By destroying the fundamental cell structure through repeated humidification/drying steps, additional avenues for escape of extremely high-pressure gas are created, thereby reducing the stresses exerted on the nutmeat during depressurization. This, coupled with possible added plasticity imparted to the nutmeat by the additional humidity in it, lowers the amount of stress acting on the peanuts, and thus ameliorated the breakage problem.
Destroying the fundamental cell structure in the manner described by Passey et al., however, results in a mealy texture being imparted to the peanuts, as is well understood by anyone familiar with the effects of hydration/dehydration cycles on food texture. Thus, the peanuts defatted by a supercritical carbon dioxide extraction process would be characterized by a mealy texture which would distract from their appeal to the consumer.
Additionally, the fat remaining in the reduced-fat product produced by the supercritical carbon dioxide extraction process does not necessarily have the same chemical composition as the fat originally present in the starting nuts. According to Biernoth et. al. U.S. Pat. No. 4,504,503, supercritical carbon dioxide and supercritical propane are very useful solvents for fractionating triglyceride fats according to their molecular weight and, more particularly, their carbon number. The temperature and pressure range employed for the extraction of oil from peanuts by Passey et al. are virtually the same as those claimed by Biernoth et al. for a fractionation effect to occur. Similarly, Biernoth et al. teach that under supercritical conditions propane likewise exhibits fat selectivity based on carbon number. From the solubility and selectivity standpoint, it would thus be advantageous to carry out extraction of peanuts and the like with a solvent under conditions exhibiting complete mutual solubility with triglycerides to avoid a selective or preferential removal of the fats. While this is not difficult to achieve when liquid PBI is used as a solvent (indeed as is the case in the present invention), it is not possible to do for liquid carbon dioxide because this material does not exhibit complete mutual solubility with triglycerides. Indeed, Friedrich et. al U.S. Pat. No. 4,493,854 teaches that carbon dioxide can be made completely mutually soluble with triglycerides; only under supercritical temperatures and pressures greater than about 9,000 psi, well above the pressures contemplated by Passey et al.
The essential object of the present invention is a process for extracting with a solvent from oil-containing nutmeats, such as peanuts and the like, a substantial quantity of oil contained therein while preserving intact their basic structure, shape, and function as human food ingredients so that the physical and textural characteristics of such nutmeats, apart from a reduced oil content, are nearly indistinguishable from those of the original raw material.
Another object of the invention is a process for obtaining by solvent extraction an at least partially defatted product as the principal product produced, with the fat or oil separated therefrom being merely a by-product, albeit an economically valuable one.
Another object is a solvent extraction process which is capable of removing oil without any discrimination on the basis of carbon number or degree of saturation of the oil and yields a solid product which contains a reduced oil content identical in constituency to the oil in the starting material.
A further object is a solvent extraction process for triglyceride containing starting material which is effective to remove the various different triglycerides in equal amount, so that the fatty profile of the removed oil as well as the solid residue is the same, other than a proportionate reduction in quantity, as the fatty profile of the starting material.
A still further object is a solvent extracted nutmeat product of significantly reduced oil content: compared to the untreated nutmeat but of otherwise nearly identical physical characteristics and fatty acid triglyceride profile.
Another object is a process for extracting nutmeats with a substantial increase in output by using as a pre-treatment step a standard roasting technique.
Yet another object is a solvent extraction process using liquefied normally gaseous solvents which are generally recognized as safe for use as food additives and which are capable of dissipating virtually completely from the extracted residue when the latter is exposed to ambient air, temperature and pressure, leaving the extracted residue nearly indistinguishable from the original material being extracted.
It is an additional object of the invention to extract from nutmeats at least 25%, and more typically 40-80%, by weight of the fat or oils originally present therein under an extraction pressure which is {fraction (1/100)}th to {fraction (1/10)}th the pressures used in the art (Passey et al), and with less than one-fifth the amount of extraction solvent.
My invention flows from two fundamental and rather surprising discoveries: The first was that certain liquefied normally gaseous hydrocarbons have the capacity not only to act as solvents to dissolve oil but to penetrate cell walls of natural cellular oil-containing materials of appreciable size and cause oil to diffuse through the resultant broken cell walls into a surrounding continuous liquid solvent phase.
The second discovery is that these solvents being normally gaseous under room temperature and pressure have the capacity when exposed to such temperature and pressure for a reasonable time to dissipate and escape from the extracted material, even where the particles have a size equal to tens of thousands of cell diameters, in contrast to normally liquid solvents such as hexane which are virtually impossible to remove without the use of such high temperatures as would render the product virtually inedible due to burning or scorching.
The surprising nature of these discoveries can be best illustrated by reference to current technical literature. As pointed out above, comminution has long been the technique of choice for ensuring that oil-containing cellular products will give up their oil. From nearly the beginnings of extraction technology, it has been recognized that the availability for extraction of a solute from a cellular product is severely restricted unless the product is completely comminuted.
This is very clearly illustrated for the case of peanuts by the original work of Goodrum and Kilgo xe2x80x9cPeanut Oil Extraction with SC-CO2: Solubility and Kinetic Functionsxe2x80x9d, Trans. ASAE 30(6):1865-1868) (1987) where extractability of oil from peanuts in high pressure carbon dioxide is shown as a function of particle size. Under the conditions studied in that work, peanut halves of average diameter 10 mm yielded well under 7% oil and, significantly, a plot of this data approached an asymptote close to that value of about 7%, thereby clearly suggesting that the peanuts would yield very little more oil even if left in solvent indefinitely. Similarly, smaller peanut pieces of average diameter 3.35-4.75 mm showed an asymptotic approach at a maximum oil recovery of approximately 35%.
These authors offer the time-honored hypothesis that only surface oil is available unless the cellular structure of the product being extracted is destroyed, citing also work by Snyder et. al. (Effect of moisture and particle size on the extractability of oils from seeds with supercritical CO2. JAOCS 61(12):1851-1856) (1984). The latter workers obtained an increase in yield of oil extracted from soybean flakes of 66% to 97% by decreasing the flake thickness from 0.81 to 0.10 mm. In fact, Snyder et al also show asymptotic behavior as the amount of oil recovered nears these maximum values for the particle sizes in question. It was noteworthy that 90% of these maximum values was reached fairly rapidly. Most importantly, when cracked (granulated) soybeans were contacted with supercritical carbon dioxide, even under conditions of extremely high pressure, only a small percent of oil was extracted from the granules, denoting that only surface oil was extracted. Snyder et al. determined by examination of scanning electron microscope photos that the cellular structure of these granules remained substantially undisturbed after exposure to high pressure gas.
With the present invention, using liquid propane, butane, and/or isobutane solvent at approximately room temperature and corresponding vapor pressure, nearly 80% by volume of original oil has been removed from peanut halves, while for peanut pieces in the size range cited above, substantially complete extraction (residual fat content below 2%) has been achieved. It was concluded that given enough contact time and sufficient solvent, much more oil could be removed than the prior art previously believed.
To state the inventive results differently, blanched peanut pieces of xe2x85x9xe2x80x3 average particle have by the invention have had 98% of the oil removed therefrom. That is, 800% more oil has been removed from blanched peanut halves while preserving intact the fundamental cell structure thereof than was previously reported possible by Goodrum, for example.
Finally, compelling evidence of the surprising nature of the present invention in its ability to remove oil from products of substantially undisturbed cell structure is the many published articles which state that with either hexane or carbon dioxide, oil in solution is not transported through unbroken cell walls, and only surface oil is removed. In the light of this consensus, it is most surprising to find that liquefied propane, butane, and isobutane under pressure has the ability to disrupt cell walls of coherent cellular oleaginous materials sufficiently for effective extraction to occur.
Electron micrographs of raw as well as roasted full-fat peanuts, show that they consist of a matrix web of contiguous cell walls, within which a continuous oil phase (exhibiting a milky appearance on the electron micrograph) containing globules of protein can be seen. The cell walls appeared as an irregular web or membrane which were substantially continuous and without breaks. Electron micrograph of a partially defatted peanut revealed a matrix of cell walls, within which was empty space except for globules of protein. The milkiness which was associated with the presence of oil within the boundaries of the cell was absent. The walls, moreover, showed discontinuities and were somewhat distorted in shape, as compared with the original material. But a web-like structure of cell walls, albeit with holes in them, remained evident. This minimal damage to cell walls may explain how the natural texture of the products was retained in the material processed by this invention.
The present process has an important economic advantage over many prior extraction systems in carrying out extraction and desolventization in a single chamber or vessel, This is a cost saving in itself, given the requirement of a treatment vessel capable of withstanding relatively high pressure. Moreover, only two manipulative physical steps need be carried out on the solid substrate (nutmeats), namely, (a) introduction into the pressure chamber, and (b) removal from the pressure chamber. The invention hence avoids excessive handling of the fragile nutmeats and minimizes the risk of damage thereto. The resistance of peanuts and other nutmeats to the release of absorbed solvent immediately upon warming requires a slower desolventization process which maximizes the driving force for volatilization of PBI vapors from the particles (without making it so high as to break the particles), followed by vacuum removal of solvent in one or more stages, followed by flushing of the extractor with inert gases to facilitate degassing of the defatted solids (i.e. removal of significant solvent residue) and safe unloading of the extractor.
Original Experiment
The genesis of this invention was pure serendipity, being the result of chance in the course of a routine continuous extraction experiment. In this experiment, 300 lbs of raw, blanched peanuts containing about 50% oil were placed in an extraction chamber, which was then sealed and liquefied butane at a pressure of about 60 psi and ambient temperature was passed up-flow through the chamber at a rate of approximately one 300 lb/hr (equivalent to 1 lb/lb peanut-hr) with the objective of making a reduced-fat peanut. After the first hour of extraction, about 10 lbs of oil were recovered and after the second hour, another 6 lbs of oil were recovered. In the third hour, only about 3 lb of oil was recovered, and for each of the successive eight hours less than 1 lb of oil was removed from the peanuts. Thus, for an extraction time of 12 hours as had been planned, as total of only about 24 lbs of oil (about 8%), were recovered. At the end the rate at which oil was being extracted was too low to measure using weighing equipment on hand; it was estimated that the final micella contained only about 0.1% extracted oil.
On its face, the experiment was an obvious failure because there appeared to be no hope of removing any relatively large amount of the oil in the original peanuts. Ordinarily, the experiment would have been terminated by draining solvent from the extractor and then desolventizing the defatted solids by blowing warm solvent vapors through them. But, as it was late at night and these operations would have consumed an additional two hours or longer, it was decided simply to consider the experiment a failure, allow the system to stand overnight, and do the draining and desolventizing on the following day,
On draining the extractor the following morning, one might have reasonably expected a small amount of oil, perhaps a pound or two, to be recovered from the weak micella after overnight contact. But when the overnight micella was; routinely de-solventized to recapture the solvent, it was discovered quite surprisingly that an additional 13 lbs of oil was recovered. In view of this unexpected development, it was decided out of pure curiosity to re-fill the extractor with fresh solvent and see what would happen. That evening, the extractor was drained, and upon de-solventizing another 10 lbs of oil were recovered. The extractor was re-filled and again left overnight, and when drained the next morning it was found that the micella contained 7 more lbs of oil. Another two filling/draining cycles produced 6 and 5 lbs of oil, respectively. Two final filling/draining cycles on the fourth day produced 4 lbs and 4 lbs of additional oil, respectively. All together, after about 96 extraction hours, 73 lbs of oil had been removed from the raw material, which was almost one-half the available oil and significantly more than the originally desired 25% reduction.
Reflection on the results of the original experiment led to the remarkable conclusion that even at the low treatment pressure liquefied hydrocarbon solvent had the power to slowly damage cell walls to such an extent that the solvent penetrated into the cells, dissolved the fat contained therein, and diffused through the damaged cells walls over a distance equal to thousands of cell diameters and into the bulk liquefied hydrocarbon bath. Although the extraction process was slow by prior art standards, what was most surprising was that so great a proportion of the oil could be extracted at all without major physical disruption of the peanuts to expose the oil to the solvent.
The desolventizing step revealed a second interesting discovery from that initial experiment. Normally, desolventizing of a flaked or comminuted product is carried out by slowly de-pressurizing the gravity-drained extractor, the initial expansion of some of the liquefied solvent creating a chilling effect on the drained bed of solids (or marc). Then, warm solvent vapor is blown through the bed to evaporate the cold liquefied solvent remaining in the marc. Finally, a vacuum is briefly applied to the extraction chamber to aspirate remaining vapors therefrom. Complete desolventization is indicated by a relatively rapid increase in temperature of desolventizing vapors leaving the extractor.
In the original experiment, the partially defatted peanuts were desolventized as stated above, and it was found that the temperature of vapors leaving the extractor rose surprisingly quickly (from which one might assume that the treated peanuts had retained very little solvent after gravity draining). On the belief that the desolventization was complete, the extractor was opened and unloaded. Quite unexpectedly, the unloaded peanuts gave off such massive amounts of solvent vapor that hydrocarbon vapor detection alarms provided for safety reasons were actually set off and, needless to say, unloading was most unpleasant. When tasted, the treated peanuts had a very strong and disagreeable solvent taste, as well as an effervescence in the mouth which presumably came from pockets of pressurized liquefied gas retained within the cellular structure of the nuts.
But in contrast to the explosion phenomenon encountered by Passey et al. in the CO2 extraction of natural peanuts, i.e. without pretreatment, the butane-extracted peanuts did not explode unless subjected to high heat. Apparently, the high vapor pressure of carbon dioxide (750 psi) at room temperature is sufficient to explode the peanuts, while the more moderate vapor pressure of butane (100 psi at desolventizing temperatures) was not sufficient to cause the same effect.
Some of the treated peanuts were transferred to a small oven and heated to 300 degrees F. in the hope that this temperature would drive the solvent out of the nuts. At this high temperature level, some of the nuts in the oven did explode. Others did not explode or even change shape, but when tasted after the heat treatment, a strong solvent taste and occasional effervescence could still be detected in many.
When elevated temperature failed to remove solvent from the peanuts, they were set aside in a woven bag pending final disposal. It was found, however, that the nuts slowly released the solvent without external measures. The vapor pressure of the normally gaseous solvent was evidently sufficiently high to in time force its way out of the nuts. Prior to being discarded the following day, the nuts were tasted again and found to have significantly less solvent flavor than before. After three days at room temperature in a woven bag, no detectable residual solvent flavor remained in the nuts.
Visual examination of the nuts revealed that in contrast to the very light yellow color of a full-fat (unextracted) blanched peanut, the surface coloration of a partially defatted peanut was cream colored or nearly white. When the partially defatted nuts were broken into pieces for interior examination, it was found that the lighter color extended entirely through their interior. This fact supported the conclusion that the solvent had succeeded in penetrating to the heart of the interior of the nuts and in removing fat from cells far away from exposed surfaces.